Casings are typically cemented into wellbores by circulating a cement slurry through the inside of a casing, out the bottom of the casing and up the annulus between the outside of the casing and the wellbore until a cement slurry level outside the casing is reached to which the wellbore is to be cemented. The cement then hardens to form a seal around the casing. Because the column of cement slurry must be fluid until the last of the cement slurry is forced into the annulus around the casing from the bottom, this method requires that the cement slurry is of a density that does not exceed the hydraulic fracture gradient of the formation around the wellbore. If this gradient is exceeded, the formation can fracture and cause the cement to be lost into the fracture. A cement slurry of a density that exceeds the formation hydraulic fracture gradient may be desired because such slurries can have greater mechanical strength, better bonding to the casing and the formation, better tolerance to elevated temperatures and greater thermal conductivity.
Further, the cement slurry must be of a density that is great enough to provide a wellbore pressure that exceeds the formation pore pressure to prevent formation fluids from invading the wellbore and interfering with the setting of the cement. It is occasionally difficult to match the density of the cement slurry to the range of densities that will satisfy these requirements.
To prevent lost circulation, when it is desirable to use a cement slurry that has a density that exceeds the fracture gradient of the formation, the cement slurry can be placed in stages directly into an annulus between the casing and the formation using a coiled tubing. An apparatus for injection of a coiled tubing into such an annulus is disclosed in, for example, U.S. Pat. No. 4,673,035. Placement of cement slurry in stages is time consuming because each stage must gel before a stage can be set above it. This makes placement of cement in stages very expensive due to equipment rental costs and the delay in completion of the well.
Conventional placement of cement from the bottom of the casing and up the annulus requires that the cement set relatively slowly because the entire annulus must be filled with cement slurry before the first cement placed in the annulus starts to become hard. When the formation within which a casing is to be cemented causes significant water loss from the cement slurry, the top of the column of cement will settle a significant amount between the time the cement slurry is placed and the time the column of cement slurry is fully hardened. This settling can be attributed to water loss from the cement slurry. Water loss additives can be added to the cement slurry, but water loss additives can be expensive and some settling will typically occur even when water loss additives are included in the cement slurry. Water loss alters the chemistry of the cement slurry resulting in inconsistent and suboptimal set cement properties. The final height of the cement is also unpredictable.
Injection of cements and curing agents through separate conduits within a casing is disclosed in, for example, the abstract of Russian Patent No. 465,583. This Russian patent abstract discloses such a method in order to provide a quickly setting cement in permafrost conditions.
Separate injection of grouts and curing agents through conduits within the casing is disclosed in U.S. Pat. Nos. 4,302,132 and 4,449,856. These grouts are intended to fill voids and thief zones within a formation with a quickly setting grout. The methods of these patents could not be used to place cement in a significant length of wellbore annulus because they are discharged from the bottom of the casing and will become hard before a significant portion of the annulus could be filled.
It is therefore an object of the present invention to provide a method to place cement in a wellbore wherein the cement hardens sufficiently fast that significant water loss from the cement does not occur. It is a further object of the present invention to provide such a method wherein the cement can be placed in a formation that has a hydraulic fracture gradient significantly less than the static head that would be formed by the cement slurry. It is another object to provide such a method wherein the cement can be placed over an extended length of the wellbore in a single continuous operation.